Method of forming multiplexed magnetic bubble detectors

ABSTRACT

A multiplexed magnetic bubble detector is described which includes a pair of detectors each having a propagation element and an underlying detector element. In one of the detectors the detector element is at the leading edge of the propagation element and in the other the element is at the trailing edge of the propagation element. The outputs from the detector elements are coupled in a bridge circuit with dummy detector elements.

This is a divisional of application Ser. No. 229,345 filed Jan. 29,1981.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of magnetic bubble detection.

2. Prior Art

In the fabrication of magnetic bubble (single wall domain) devices,particularly magnetic bubble memories, it is important to limit thenumber of connections that must be made between the memory substrate andexternal circuits. Also, it is advantageous to use as few as possibleexternal circuits and to have, for example, the same driving circuitpower two bubble generators. In copending application Ser. No. 25,848,filed Apr. 2, 1978, now U.S. Pat. No. 4,272,817, entitled "MultiplexedSingle Wall Domain Generators," (assigned to the assignee of the presentapplication), a system is descriped for multiplexing bubble generators.As will be seen, the present invention teaches an apparatus formultiplexing the outputs of bubble detectors.

There have been attempts in the past to multiplex bubble detectoroutputs. In one scheme, four chevron detectors are formed at differentangular orientations. For example, the detectors are oriented each 90degrees from the other, like spokes of a wheel. Then, the detectors arecoupled to a common sensing circuit. One problem with this scheme isdescribed in conjunction with FIG. 3a of the specification.

Another aspect of the present invention is a method for optimizing thesignal level output from each of the multiplexed bubble detectors. Oneprocess for adjusting the signal level from a magnetic bubble detectoris described in copending application Ser. No. 163,574 filed June 27,1980, now U.S. Pat. No. 4,300,209, entitled "Method for Adjusting SignalLevel Output from Magnetic Bubble Detector," assigned to the assignee ofthe present application.

SUMMARY OF THE INVENTION

A magnetic bubble detection apparatus is described which includes afirst and second magnetic bubble detector. The first bubble detectorincludes a first propagation element which propagates bubbles from itsleading edge to its trailing edge under the influence of a rotatingmagnetic field. This first detector also includes a first detectionelement disposed in the region of the leading edge of the firstpropagation element. A bubble passing through the detector is detectedas it is transferred along the leading edge of the first propagationelement. The second magnetic bubble detector also includes a propagationelement which propagates bubbles from its leading edge to its trailingedge under the influence of the rotating magnetic field. A seconddetector element is disposed in the region of the trailing edge of thissecond propagation element. A bubble being propagated by the secondpropagation element is sensed when the bubble is at the trailing edge ofthe second propagation element. In this manner, the outputs from thefirst and second detector elements may be readily multiplexed since theyoccur at different times during each cycle of the rotating magneticfield.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a prior art magnetic bubble detector.

FIG. 2 is a cross-sectional elevation view of a portion of the detectorof FIG. 1 generally taken through section line 2--2 of FIG. 1.

FIGS. 3a-3c illustrate various detector elements and the outputs fromthe elements.

FIG. 3a illustrates a chevron detector and the output from thisdetector.

FIG. 3b illustrates a bubble detector which includes a separate detectorelement, and the output from this element when the element is disposedin the region of the leading edge of the propagation element.

FIG. 3c illustrates a bubble detector which includes a separate detectorelement, and the output from this element when the element is disposedin the region of the trailing edge of the propagation element.

FIG. 4 is a schematic of the detection apparatus of the presentinvention, in its presently preferred embodiment.

FIG. 5 is a cross-sectional elevation view illustrating processing stepsused to define the detector element used in the present invention and tooptimize its output.

DETAILED DESCRIPTION OF THE INVENTION

A magnetic bubble detection apparatus is described which providesmultiplexed outputs from two separate detectors. With the inventedapparatus two independent streams of bubbles are detected with separatedetector elements and the resultant signal from these separate elementsis sensed on a single pair of lines. This reduces the number of packagepins required, and moreover, the number of sense amplifiers needed fordetection.

In the following description numerous specific details are set forthsuch as specific thicknesses, etc. in order to provide a thoroughunderstanding of the present invention. It will be obvious to oneskilled in the art that the present invention may be practiced withoutthese specific details. In other instances, well-known structures andprocesses are not set forth in detail, in order not to obscure thepresent invention in unnecessary detail.

Referring now to FIG. 1, a prior art bubble detector is illustratedwhich includes an expander section 10, a propagation element 12 whichcomprises a column of chevrons, a bubble trap defined between chevroncolumns 13 and 14, and an additional column of chevrons 15. A magneticbubble, as it is propagated past the element 12, is detected by anelongated detector element (strip of magnetoresistive material) disposedbelow element 12. As is generally the case, magnetic bubbles are movedinto the detector of FIG. 1 in the direction 20 under the influence ofan in-plane rotating magnetic field. As the bubbles are propagated pastthe column of chevrons 12, the resistance of the underlying detectorelement is changed, and this change is sensed. Bubbles are thenpropagated onto element 13 and become trapped between the elements 13and 14.

An additional column of chevrons 15 which is identical to the column ofchevrons 12, and an additional "dummy detector" element 17 which isidentical to the detector 16 is disposed adjacent to the trap. Eventhough elements 15 and 17 do not receive and sense bubbles, they areaffected by the rotating magnetic field. The signal from the element 17is used as a reference signal to effectively cancel (reject) the effectsof the rotating magnetic field on the element 16.

In FIG. 2 only those layers of a memory or other device needed to viewthe detector of FIG. 1 are shown. The structure is formed on a substrate21 which is typically a garnet substrate. An ion implanted magneticgarnet (epitaxial layer 22) is employed on the substrate for themagnetic bubble storage layer. The single wall domains are moved in thislayer as is well-known. A silicon dioxide layer 23 which isapproximately 1500 A thick is formed over the epitaxial layer. Then thedetector element 16 is fabricated on the layer 23 using ordinaryphotolithographic techniques from a magnetoresistive material such aspermalloy. The detector element 16, in the presently preferredembodiment, is approximately 2 microns wide and approximately 500 Athick. An additional oxide layer is formed over layer 23 to insulate thedetector 16 from the overlying permalloy members. This oxide layer 24 isapproximately 1500 A thick. Then, the permalloy propagation elementssuch as the column of chevrons 12 are formed over the oxide layer 24 ina well-known manner.

The present invention may be best understood with reference to thediagrams of FIG. 3. In FIG. 3a, a prior art chevron detector 25 isillustrated which comprises a plurality of interconnected chevrons. Whena bubble is moved through this chevron detector under the influence of arotating magnetic field, the waveform shown to the right of detector 25results for each cycle of the rotating magnetic field. As can beappreciated, it is difficult to multiplex a number of outputs fromdetectors such as detector 25 because of the resultant waveform.

In FIG. 3b, a detector similar to the detector shown in FIG. 1 isillustrated, however, the detector element 27 is disposed in the regionof the leading edge of the propagation element 26. Assuming bubbles aremoving in the direction of arrow 29, they first reach a first edge ofthe element 26 (leading edge) and then are propagated to the oppositeedge (trailing edge) of the element. For the relative location of thedetector element 27 and propagation element 26 of FIG. 3b, a bubble issensed by the detector during the first part of each cycle of therotating magnetic field as shown to the right of elements 26 and 27(ignoring the effects of the rotating magnetic field on the detectorelement).

In FIG. 3, another propagation element 30 is illustrated with anunderlying detection element 28. This time, however, the detectionelement 28 is disposed in the region of the trailing edge of the element30. In this position, a bubble is sensed towards the end of each cycleof the rotating magnetic field as indicated by the graph shown to theright of elements 28 and 30.

The appreciation that during a cycle of the rotating magnetic field abubble can be sensed at different portions of this cycle (compare FIG.3b and FIG. 3c) is put to use for multiplexing purposes as taught by thepresent invention.

Referring now to FIG. 4, in the presently preferred embodiment, thedetector elements of two detectors are coupled in a bridge circuit.Detector 1 of FIG. 4 includes an expander section not illustrated whichexpands bubbles and propagate them into propagation element 32 (a columnof chevrons). The bubbles are then propagated into a trap which isdisposed between the column of chevrons 32 and the columns of chevrons35. A magnetoresistive detection element 33 is disposed beneath thepropagation element 32 in the region of the leading edge of the element32. An identical element, dummy element 34, is disposed in the region ofthe leading edge of the column of chevrons 35. Element 33 iselectrically coupled between contacts 45 and 47, and the dummy element34 is coupled between the contacts 46 and 49. Referring briefly to FIG.2, for the detector 1 of FIG. 4, the detector element 33 and dummyelement 34 would be centered about the line 36 and again insulated fromthe overlying chevrons.

Detector 2 of FIG. 4 again includes an expander section not illustratedwhich propagates bubbles into the propagation element 40. Disposedbeneath the propagation element 40 is a detector element 41 which againcomprises a magnetoresistive material. The element 41 is coupled betweenthe contacts 48 and 51. The element 41 is in the general region of thetrailing edge of the propagation element 40. A trap is disposed betweenthe propagation element 40 and the column of chevrons 42. A dummyelement 43 is disposed beneath the column of chevrons 42 in the regionof the trailing edge of chevrons 42. This dummy element is connectedbetween contacts 50 and 52. Referring briefly to FIG. 2, the detectorelement 41 and element 43 would be centered on line 37 for detector 2 ofFIG. 4. (Note that the precise location of the detector elements anddummy elements relative to the leading edge and trailing edge of thepropagation element is not particularly critical.)

The layout (in terms of size and orientation relative to the rotatingmagnetic field) of elements 32 and 33 when compared to elements 35 and34, respectively, is identical to provide proper electrical performance.The same is true for elements 40 and 41, and elements 42 and 43,respectively. (This, of course, is true for the prior art detector ofFIG. 1).

The detector 1 and detector 2 of FIG. 4 are coupled in a bridge circuit.A potential is applied to the contacts 44 and 46. Contact 47 is coupledto contact 50, and contact 48 is coupled to contact 49. The contacts 51are coupled to ground. In this bridge circuit, the dummy elements nullout the effect of the rotating magnetic field on the detector elementsand allow the bubbles passing through the propagation elements 32 and 40to be sensed. The output signal from the detectors is taken fromcontacts 47 and 49 which contacts are typically coupled to package pins,and via lines on a printed circuit board to a sense amplifier.

By knowing the portion of a cycle of the magnetic field during which abubble is detected (or not detected) the output from each of thedetectors 1 and 2 is known.

Referring again to FIG. 3, the output sensed from the bridge circuit ofFIG. 4 (and at the sense amplifier) consists of a graph of FIG. 3bsuperimposed on the graph of FIG. 3c. It is apparent that the signalcaused by a bubble moving through element 32 of FIG. 4 can readily bediscerned from a bubble moving through element 40 even though thebubbles move through these column of chevrons at the same time. The"quiet period" (no signal) region of FIG. 3c falls within the periodwhen a signal is generated in FIG. 3b, and vice versa.

It has been found that the output from the detector elements such asdetector elements 33 and 41 of FIG. 4 can be optimized by having thewidth to thickness ratio of these elements equal to a predeterminednumber. By way of example, with a prior art chevron detector (FIG. 3a),output signals of approximately 5 mv are typical. By controlling the w/tratio of the detector elements used in the present invention, outputsignals of approximately 20 mv are obtained. (This assumes the length ofchevron detector is approximately equal to the length of the element 33or 41.

Through empirical determination it has been found that the ratio w/t forthe detector element (see FIG. 5) should be approximately equal to 100for maximum output where a typical permalloy is used. (In the presentlypreferred embodiment, the permalloy comprises 81% nickel and 19% iron).Due to process variations the w/t ratio can vary as much as +20%. Thesevariations, particularly at the extremes, substantially deteriorate theoutput from the detector elements.

During fabrication in the presently preferred process, the layer fromwhich the detector element 61 of FIG. 5 is etched, is first formed overthe oxide layer 60. (The detector element 61 corresponds to the elements33,34, 41 or 43 of FIG. 4.) Ideally, as presently implemented, thislayer should be 500 A thick (assuming w=2 microns and t=500 A). Afterthe fabrication of the layer, its thickness is checked, for example, bychecking the sheet resistance of the layer. Once the thickness is known,the desired width is likewise known since the w/t ratio should beapproximately 100. A photoresist layer is formed over the permalloylayer and with an ordinary masking step, a masking member 62 is formedwhich is used to define the detector 61. If the thickness of the layeris 500 A, the width should be approximately 2 microns. In the presentlypreferred embodiment, the width of the masking member 62 is made widerthan 2 microns, for example, 2.5 microns. Then the etching step used toetch the permalloy layer is controlled to provide an undercutting 64under the photoresist member 62. This controlled undercutting is allowedto continue until the desired width w is achieved. As is well-known,this etching can be quite precisely controlled.

The thickness of the permalloy layer is generally quite uniform over theentire wafer. Thus, all the detector elements (and dummy elements) on agiven wafer will require the same etching time and can be formed withone masking and etching step.

The same result can be achieved by first forming an extra thickpermalloy layer from which the detectors are to be etched. For example,instead of attempting to achieve a 500 A thickness, a 750 A thickness issought. Then, after the etching with a photoresist member such as member62 of FIG. 5, the width to thickness ratio is measured. For this casemember 62 has a width of approximately 2 microns and no undercutting isattempted, It, of course, would be expected that the ratio w/t is lessthan 100 since the permalloy layer was intentionally made thick. Ihedetector elements are then etched to decrease their thickness until thedesired thickness is achieved. This etching is preferably done with ionmilling with the entire wafer being subjected to such milling.

Thus, a magnetic bubble detection apparatus has been described whichpermits the multiplexing of the outputs from two detector elements. Amethod has also been described for optimizing the output from thesedetector elements.

We claim:
 1. In the fabrication of a magnetic bubble detector which includes a magnetoresistive detection element and a propagation element insulated from said detection element, a method for optimizing the output from said detection element comprising:controlling the ratio of the width and thickness of said detection element such that said ratio is approximately equal to a predetermined number; whereby the output of said detection element is optimized.
 2. The method defined by claim 1 wherein said detection element comprises permalloy and said ratio is equal to approximately
 100. 3. During the fabrication of a magnetic bubble detector which includes a magnetoresistive detection elements and a propagation element insulated from said detection element, a process for optimizing the output from said detection element comprising the steps of:determining the thickness of the layer from which said detection element is to be formed; forming a masking member over said layer to define said detection element; etching said detection element from said layer, including controlling the undercutting of said detection element below said masking member so as to obtain a predetermined width of said detector element, said width being selected such that the width to thickness ratio of said detection element is approximately equal to a predetermined number; whereby the output of said detection element is optimized.
 4. The process defined by claim 2 wherein said detection element comprises permalloy and said predetermined number is approximately
 100. 